Field of the Invention and Related Art
The present invention relates to a single lens having an aspherical surface and, more particularly, to a single lens which is aspherical on both surfaces and has a value of NA (Numerical Aperture) that is on the order of 0.42-0.50.
Recently, optical disks such as video disks or compact disks have been widely employed as information recording carriers (or media) with large storage capacity in the form of optical memories.
The optical card, an optical information recording carrier like an optical disk, has also received widespread attention due to advantages of large storage capacity and handy transportability.
In order both to record information in those recording carriers with high density and/or reproduce the recorded information therefrom with high accuracy, objectives (or object lenses) for use in information recording reproducers are required to have an optical resolution of about several .mu.m. This in turn necessitates an objective with an NA above 0.4.
When objectives are used in such applications, it is further necessary to maintain a sufficient distance between the surface of a recording carrier, such as an optical disk or an optical card, and the objective, thereby to prevent contact therebetween and to avoid damage to the recording carrier and the objective.
Moreover, most of the information recording reproducers have been designed to move objectives along the optical axis or in a direction perpendicular to the optical axis as part of a servo control of auto-focusing or auto-tracking. Therefore, the objectives used to this end are required to be smaller in both size and weight for the improved responsivity.
In the past, Japanese Patent Laid-Open App. No. 58-24021 (U.S. Pat. No. 4,484,802), App. No. 58-208719, and App. No. 60-122915, for example, have disclosed objectives of the above-discussed type wherein a lens system compris four groups of four lenses.
However, the objectives as disclosed in the above published applications have a large overall length of the lens system and cannot achieve a decrease in both size and weight.
To overcome the above-mentioned drawbacks, development of aspherical single lenses has been made intensively and extensively. Relevant techniques are disclosed in the former publications of U.S. Pat. Nos. 4,027,952, 4,449,792, 4,571,034, 4,743,093, as well as in Japanese Patent Laid-Open App. No. 57-201210, App. No. 58-68711, App. No. 59-23313, App. No. 61-11715, and App. No. 61-56314, for example.
The aspherical single lenses as disclosed in such further publications are designed to be suited for the specifications of optical disks, but are quite unsuitable for use in recording carriers such as optical cards. Optical cards have protective layers coated on the information recording surfaces that are thinner than those on the optical disks.
Since optical cards typically have a thickness on the order of 0.8 mm, which is almost the same as that of commonly used magnetic cards, a transparent protective layer of the optical card should have a thickness t of about 0.4 mm, taking into account structural strength and other factors of the optical card.
In accordance with the embodiments of aspherical single lenses as disclosed in the above further publications where the focal length of the lens is F, the thickness t of a transparent protective layer of the recording carrier then is state to be on the order of 0.26 F-0.28 F, for those lenses exemplified in the embodiments that would be adapted to optical cards. Accordingly, the focal length F of the objective should become 1.43-1.54 mm, for a thickness t= 0.4 mm of the transparent protective layer coated on the optical card.
However, such a design is not practical because of the disadvantages that the radius of curvature is too small, making manufacture of the objective very difficult, and a region (or image height) in which good imaging characteristics are obtained at diffraction limits is very small.
One feasible practical means of solving the above disadvantages is to interpose a parallelepiped plate between the objective and the optical card in order to compensate for the thickness of the protective layer. For example, the focal length of an objective in the prior art is set to F=4.5 mm for facilitating manufacture of the objective. The thickness of the protective layer theoretically necesarry for that setting is 1.17-1.26 mm, whereas the protective layer actually coated on the optical card is only 0.4 mm thick. As a result, what is needed is to interpose a parallelepiped plate having a thickness on the order of 0.77-0.86 mm, or a thickness equivalent to the difference therebetween.
However, this solution is not preferably because it contradicts the requirements of improved capability due to smaller size and weight of an optical system, and lowered cost due to a reduced number of parts.
In addition, the aforesaid disadvantages will be equally encountered in optical heads employed in optical information recording reproducers for recording and/or reproducing information, if a lens as disclosed in the above-stated embodiments is used as a collimating lens to collimate the divergent light beam emitted from a semiconductor laser, as a light source, with high efficiency and small loss of the amount of light. In other words, a parallelepiped plate of glass usually mounted on a semiconductor laser for protection has a thickness t on the order of about 0.25-0.35 mm.